Method of surface hardening of metal articles



Dec. 30, 1952 T. M. l. NOREN 2,623,836

METHOD OF SURFACE HARDENING OF METAL ARTICLES Filed Feb. 2'7, 1948 v INVENTOR fire aml'rzn/Mrezg.

TIORNEYS Patented Dec. 30, 1952 METHOD OF SURFACE HARDENING OF METAL ARTICLES Tore Mans Ivan Norn, Gothenburg, Sweden, as-

signor to Elcktriska Svetsningsaktiebolaget, Gothenburg, Sweden, a corporation of Sweden Application February 27, 1948, Serial No. 11,394 In Sweden January 29, 1947 Section 1, Public Law 690, August 8, 1946 Patent expires January 29, 1967 Claims.

The present invention relates to a method of surface hardening metal articles by induction heating and subsequent rapid cooling, and to apparatus for carrying out said method. More specifically, the invention relates to an improved method of and apparatus for effecting the rapid cooling, or quenching, required in the general method above referred to.

It is an object of the invention to provide an improved surface hardening method which is not liable to cause hardening cracks in the surface of the material and which is yet capable of yielding a satisfactory degree of hardness.

Another object of the invention is to provide an improved method of surface hardening articles of steels or other alloys susceptible to surface cracks while at the same time requiring a high rate of cooling in order to suppress an undesirable structural chance otherwise liable to occur in a certain temperature interval during cooling.

Still another object of the invention is to provide an improved method of surface hardening steel resulting in a treated surface having an average hardness comparable to or greater than the one produced by conventional water quenching, but substantially free from the soft spots frequently encountered in conventional induction hardening practice.

According to the principal feature of the invention, cooling is effected by subjecting the article to be quenched first to a current of gaseous cooling medium, for example air, for a period sufiicient to cause the temperature of the surface zone of the article to fall to a predetermined value, and immediately thereafter sub- J'ecting the article to the action of a liquid cooling medium, for example water.

The process just indicated is particularly suitable for, but is not limited to, the treatment of such steels or other alloys in which an undesirable structural modification will occur in a certain temperature interval during the quenching period, if the cooling rate is below a definite minimum rate exceeding the one attainable by the use of gaseous cooling medium. In such cases the process according to the invention may be conducted in such manner, that the surface to be hardened is subjected to the action of the liquid coolant as soon as the temperature of the surface zone has fallen to a value exceeding by a predetermined amount, for instance between 0,and 100 (3., the upper limit of the interval aforesaid.

As an instance of alloys to which the surface hardening method according to the invention is particularly applicable, asteel having the follow- 2 ing approximate composition might be mentioned:

Percent C 0.30 to 0.35 Mn 0.70 to 1.00 Cr 0.90 to 1.20

Rest substantially iron.

Such steels hitherto have been considered to be unsuitable for surface hardening, as on one hand the customary water cooling is liable to cause hardening cracks, on the other hand air cooling has proved not to yield the required hardness. In surface hardening a steel of the kind stated, quenching according to the invention may be carried out by subjecting the treated surface first to a current of gaseous cooling medium, for a period of from 3 to 15 seconds which is suificient to cause the temperature of the surface zone of the article to fall to a value between 400 and 600 0., preferably between 400 and 500 C., and immediately thereupon subjecting the article to the action of a liquid cooling medium, for example Water, for a period sufficient to cool the article to room temperature or to any higher temperature from which the article may be allowed to cool in still air.

The improved results rendered possible by the above method might be explained as follows. The purpose of the quenching operation is to produce a hardened surface zone, or case, consisting substantially of martensite. If the rate of cooling from the quenching temperature is too slow, however, other components, namely pearlite and bainite, will be produced instead of or in addition to the martensite. Thus, pearlite is produced if the surface zone is allowed to remain too long at temperatures between 600 and 700 C., (approximately) while bainite is produced if the surface is allowed to remain too long at temperatures between 400 and 500 0. (again approximately). In steels of the particular kind above referred to (chromium-manganese steels), even a comparatively low cooling rate, however, is sufficient to suppress the pearlite formation. This explains why it is possible to use cooling by air (or other suitable gaseous coolant) in the first stage of the quenching operation which requires from 3 to 15 seconds. In the temperature interval of the bainite formation, the critical cooling rate required for suppressing the bainite formation is substantially higher than the one attainable by the use of a gaseous coolant. According to the invention, however, the article to be hardened is subjected to the action of a liquid coolant during the second stage of the quenching operation, resulting in suppression of the bainite formation as well. Thus, it has been possible to 3 obtain a substantially martensitic structure while avoidingthe risk of crack formation caused by the use of conventional water quenching.

It should be expressly stated, however, that the invention is not limited to any specific theory regarding the ultimate physical reasons of the improvement attained, nor to the specific temperature limits indicated in the aboveexplanation.

The method according to the invention will result not only in the elimination of hardening cracks, but also in an improved uniformity of hardness throughout the surface treated. In conventional induction hardening, in which the article heated is quenched from heatingtemperature, for example 800 0., by means of a shower of water or the like, the vaporization of the water is liable to result in the formation of stationary steam pockets or bubbles which are difficult to suppress even by the use of high water pressure and which are believed to be responsible forthe frequent occurrence of soft spots, or isolated areas having considerably lower hardness than'the rest of the hardened surface. No such difficulties have been encountered with the method according to the invention. Actually, said method may yield higher hardness values than the conventional quenching method. Thus, a chromium-manganese steel of the type above referred to will attain a surface hardness of 60 to 61 a Re after quenching. with compressed air to a temperature somewhat below red heat and subsequent quenching with water, as compared to a hardness of 55 to 58 Re after conventional quenchingwith'water alone. As already mentioned, theair quench should be allowed to proceed during a period of between 3 to 15 seconds to a temperature not above 600 C. The temperaturechosen in any particular case will depend on the degree of hardness desired. case of the steel above referred to, air quenching down to room temperature will result in a hardness of 48 to 50 Re. It will thus-be possible to obtain any desired hardness between 48 to 50 RC and. 60 to 61 Baby a suitable choice of the temperature at which the water quench is caused to begin.

An apparatus for. carrying out the invention may comprise a combined single-turn inductor and cooling headmember of the general type used for conventional. induction hardening work. Said member is, as usually, connected to a source of high frequency electric current through suitable switching means and provided with a system of outlet orifices on the side facing the work, said orifices being connected through suitable ducts or cavitiesin the inductor member to a source of quenching liquid under pressure. According to the invention, the inductor member is adapted to be supplied with gaseous quenching fluid as well, for example compressed air. The source of gaseous quenching fiuid may be connected either to the ducts and orifices which serve also to receive the quenching liquid, or to a separate system of ducts and orifices. According to the invention, the quenching liquid is supplied to the inductor member at the end of a period of predetermined duration, for example 5 or 10 seconds, following upon disconnection of the inductor from the source of current. During said period, air alone is supplied to the inductor.

The air or othergaseous'cooling medium may besupplied to the inductor during the entire heating period as well. This has proved to be In the very advantageous, as any tendency to overheating of the inductively heated surface of the work is effectively checked thereby. Also it simplifies the operation of the apparatus, as no particular measures will be required to start the air cooling action on disconnection of the inductor from the electric supply. The supply of air may be allowed to continue during the water quenching period as well, so that the work will be cooled by a mixture of compressed air and water (or other quenching liquid) during the second quenching stage. Said mixture will in many cases effect a more efiicient cooling than water alone, presumably on account of the increased turbulence and improved distribution of the water effected by the compressed air. Preferably automatic means are provided for controlling the duration of the various stages of the process. For instance, a conventional time relay may be provided for operating the means for admitting the quenching liquid to the inductor. at the end of the air quenching period.

Finally,a pairof additional advantages of the new method according to the invention might be mentioned. As the air. cooling period is comparatively long, any original temperature differences between different portions of the heated surface will be substantially decreased by the time the water quench begins. Another advantage resides therein that. the process may be adapted to the peculiar hardening properties of different steels, so that it will now be possible to surface harden steels of various types hitherto not considered to be suitable for induction hardening.

The annexed drawing shows by way of examplea sectional elevation of an inductor together witha diagrammatic representation of the electric circuit and the coolant supply system connected thereto.

The inductor, which is shown in operating position surrounding an article I to be surface hardened, is of the well-known type comprising two halves or jaws connected by a hinge joint l. Each of said halves, is hollow to confine a cavity or duct the inner wall of which is provided with a plurality of orifices or nozzles 5. The lower extremity of the jaws has an extension 8, I, each of which extensions is electrically connected to one terminal of the secondary 8 of a transformer t and has also fitted therein a forked connecting piece i8, ll, opening into the internal cavity or duct. Each of said connecting pieces has one branch l2, 13 connected through conduits Hi, It and valve l! to a compressed air distributing system l6, and a second branch l8, 19 connected through conduits 20, 2! to a pump 23, or other suitable source of water under pressure. The valve 22 is controlled by a solenoid 24. in such manner that the valve is normally closed and is opened when the solenoid is energized. The solenoid 24 is connected inra circuit comprising battery 25, movable contact. 25, stationary contact 21,. switchv 23, solenoid 2d and other pole of battery 25. Contacts 26 and 21 form part of a retarded relay or time switch 29 adapted to be controlled by the current in a series circuit through high frequency generator 3%, switch 3i, a winding (not shown) of relay 29, and secondary 32 of transformer 9, in such manner that the movable contact 26 is opened as soon as said series circuit is energized and that, on de-energization of said circuit, the contact 25 is caused to return. to its normal position shown at the. end

of a predetermined period following upon the deenergization of the circuit.

The apparatus is operated in the following manner. First, the valve 11 is opened, causing compressed air to be projected towards the workpiece through nozzles 5. Then, the switches 3i and 23 are closed in swift succession and in the order named. The closing of switch 31 causes current to flow through the inductor 2, 3 to heat a surface layer of the workpiec I. The closing of switch 28 has no immediate effect, as contact 26 is opened immediately on closing of switch 3 5. At the end of a predetermined period corresponding to a desired depth of the heated surface layer of the workpiece, the switch 3! is opened again, so that the supply of energy is discontinued and the workpiece is subjected to the action of the compressed air alone. At the end of a further period determined by the retarded relay or time switch 29, during which time the cooling action of the compressed air has caused the temperature of the heated layer of the workpiece to sink to a predetermined value, c. g. 450 C., the movable contact 23 returns to normal position, whereby the solenoid coil is energized and opens valve 22, causing cooling water to be projected onto the work through nozzles 5. It will be noted that the compressed air is still on, causing an increased velocity and increased turbulence of the cooling water escaping through nozzles 3, so that a more efficient cooling action is obtained than the one which would result from the use of water alone. Finally, switch 23 is opened and the valve H is closed.

The apparatus described as well as the method carried out with the use of said apparatus may be modified in a variety of ways within the scope of the invention as defined by the fol owing claims.

I claim:

1. A method of surface hardening articles of hardenable steels of such composition that the cooling rate required to suppress formation of bainite during quenching is higher than the one required to suppress the formation of pearlite, comprising the steps of heating a surface zone of the article to a temperature within the austenite range by means of high frequency currents induced therein, quenching said heated surface zone to a temperature of about 500 C. by means of a flow of gaseous cooling medium during a time period of from 3 to seconds, and further quenching said zone to a temperature within the martensite forming range by means of a liquid cooling medium, said second quench following immediately upon the first one.

2. A method of surface hardening articles of hardenable steel-s of such composition that the cooling rate required to suppress formation of bainite during quenching is higher than the one required to suppress the formation of pearlite, comprising the steps of heating a surface zone of the article to a temperature within the austenite range by means of high frequency currents induced therein, quenching said heated surface zone to a temperature between 400 and 600 C. by means of a flow of gaseous cooling medium during a time period of from 3 to 15 seconds and submitting said zone to a second quench from said temperature to a temperature within the martensite forming range by means of a liquid cooling medium, said second quench following immediately upon the first one.

3. A method of surface hardening articles of hardenable steels of such composition that the y bu cooling rate required to suppress formation of bainite during quenching is higher than the one required to suppress the formation of pearlite, comprising the steps of heating a surface zone of the article to a temperature within the austenite range by means of high frequency currents induced therein, and submitting said heated surface to a first quench to a temperature of between 400 and 600 C. by means of a flow of gaseous cooling medium only, and to a second quench to a temperature within the martensite forming range by means of a flow of liquid cooling medium mixed with a flow of gaseous cooling medium, said second quench following immediately upon the first one, said second quench being at a rate sufiicient to form martensite and to avoid the formation of bainite.

4. A method of surface hardening articles of hardenable steels of such composition that the cooling rate required to suppress formation of bainite during quenching is higher than the one required to suppress the formation of pearlite, comprising the steps of heating a surface zone of the article to a temperature Within the austenite range by means of high frequency currents induced therein while at :the same time subjecting the surface to a flow of gaseous cooling medium, discontinuing the heating of said surface zone while continuing, for a period of from 3 to 15 seconds, the supply of gaseous cooling medium thereto until the temperature of said surface drops to about 500 C. and immediately subjecting said surface zone to a flow of liquid cooling medium for a further period of sufficient length to carry the temperature of said zone into the martensite forming range.

5. A method of surface hardening a metal article consisting at least partly of an alloy having the following composition:

Per cent Carbon 0.30 to 0.35 Manganese 0.70 to 1.00 Chromium 0.90 to 1.20

Balance substantially iron.

TORE MANs IVAN NORE'N.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,170,130 Denneen et al. Aug. 22, 1930 2,320,573 Doelker June 1, 1943 2,356,150 Denneen et al Aug. 22, 1944 2,388,231 Somes Oct. 30, 1945 2,404,987 Rudd July 30, 1946 OTHER REFERENCES Iron Age, February 4, 1943, pages 45-48, article by Shepherd entitled Martempering. 

3. A METHOD OF SURFACE HARDENING ARTICLES OF HARDENABLE STEELS OF SUCH COMPOSITION THAT THE COOLING RATE REQUIRED TO SUPPRESS FORMATION OF BAINITE DURING QUENCHING IS HIGHER THAN THE ONE REQUIRED TO SUPPRESS THE FORMATION OF PEARLITE, COMPRISING THE STEPS OF HEATING A SURFACE ZONE OF THE ARTICLE TO A TEMPERATURE WITHIN THE AUSTENITE RANGE BY MEANS OF HIGH FREQUENCY CURRENTS INDUCED THEREIN, AND SUBMITTING SAID HEATED SURFACE TO A FIRST QUENCH TO A TEMPERATURE OF BETWEEN 400 AND 600* C. BY MEANS OF A FLOW OF GASEOUS COOLING MEDIUM ONLY, AND TO A SECOND QUENCH TO A TEMPERATURE WITHIN THE MARTENSITE FORMING RANGE BY MEANS OF A FLOW OF LIQUID COOLING MEDIUM MIXED WITH A FLOW OF GASEOUS COOLING MEDIUM, SAID SECOND QUENCH FOLLOWING IMMEDIATELY UPON THE FIRST ONE, SAID SECOND QUENCH BEING AT A RATE SUFFICIENT TO FORM MARTENSITE AND TO AVOID THE FORMATION OF BAINITE. 